There is a growing demand in the communications industry for low-cost compact integrated circuits (IC) that incorporate digital and analog devices in a single chip. For instance, passive components such as resistors, inductors and capacitors can be advantageously included in radio-frequency metal oxide semiconductor (RF-MOS) circuits. The incorporation of such components, into semiconductor devices is not without problems, however.
Integrating thick metal lines of an inductor into a semiconductor device, for example, can cause parasitic capacitance between the inductor and the substrate. This, in turn, lowers the quality factor of the RF circuit, as measured by the Q-value, the ratio of the inductive reactance to its effective series resistance. Similarly, integrating the metal plates of a capacitor inside a semiconductor device can cause undesirable capacitive coupling to the substrate and increase the inherent series resistance of the plates.
One way to minimize these deleterious effects is to locate the metal structures comprising the inductor or capacitor outside of a non-conductive protective overcoat covering the IC's top surface. A non-conductive protective overcoat is typically used to prevent moisture or dirt from contacting circuit features in the IC. By removing the inductor as far from the substrate as possible and outside of the protective overcoat, parasitic capacitance can be reduced. Moreover, the placement of metal structures outside the protective overcoat advantageously uses the free area above the IC package to construct passive structures. When the passive structure is a capacitor, placement in this free area also helps lower the series resistance of the metal plates of the capacitor. Additionally, locating the inductor and the capacitor outside the protective overcoat facilitates construction of RF drive circuits outside the IC chip.
Locating metal structures at the surface of the IC can be problematic, however. For instance, certain metals are susceptible to oxidation. Oxidation increases the resistivity of the metal thereby reducing the Q-value for an inductor made of the metal. This can be problematic when, due to skin effects, the bulk of the current passing through the metal line of an inductor occurs at or near the surface of the metal line. Furthermore, metal lines, when subject to high current loads, can experience electro-migration (EM) effects. EM causes metal atoms to migrate down the metal line or between adjacent metal lines of the inductor, thereby bringing about a short circuit.
Alternatively, instead of leaving the surface metal uncovered, one may apply the non-conductive protective overcoat or encapsulating material. Unfortunately, however, such materials, as explained above, can cause a device to operate at slower than desired switching speed or have a reduced Q-value.
Accordingly, what is needed in the art is a method of protecting exposed metal structures at the surface of ICs without suffering the limitations of the prior art.